The present invention relates to the field of signal detection. More specifically, one embodiment of the invention provides an improved circuit for detecting a signal which is a pulse stream with at least some predetermined characteristics using those predetermined conditions to detect the pulse stream accurately through noise added to the signal.
Pulse detection is a well known form of signal detection. Typically, a binary (i.e., comprising logical "0"'s or "1"'s) data stream is encoded as pulses in a pulse stream. The presence of a pulse in a certain time window encodes for a logical "1" and the absence of a pulse in the time window encodes for a logical "0". In a variation of such a system, pulse widths carry the information, with a pulse's width representing a digital value. With this "pulse width encoding", there are a limited number of valid widths a pulse can have. In yet another variation, information is encoded in the falling or rising edges of a signal. Regardless of how the information is encoded, accurate detection and decoding rely on precise detection of the beginnings and/or ends of pulses.
Pulse detection is needed in, for example, modems, bar-code readers, and optically-coupled transmitter/receiver pairs. The latter includes fiber optic systems and optocouplers.
In such systems, the information is clearly encoded and the pulses are transmitted with very sharp rising and falling edges. However, during transmission, pulse edges get distorted due to channel bandwidth limitations, detection circuit bandwidth limitations and noise.
A detection circuit normally amplifies an incoming signal and then applies the amplified signal to a decision circuit. If the level of the amplified signal is below a predetermined level (the "detection threshold"), the decision circuit outputs a logical "0" as its estimation of the digital value encoded in the signal. If the signal is above the detection threshold, then the decision circuit outputs a logical "1".
An example of a known detection circuit 10 is shown in FIG. 1. Detection circuit 10 is shown comprising an amplifier 12, a peak detector 14 and a comparator 16. Waveforms at nodes 20, 22, 24, 26 and 28 of detection circuit 10 are shown in FIG. 2 as waveforms W20, W22, W24, W26 and W28, respectively. The top portion of FIG. 2 shows the original signal which, after transmission and amplification by amplifier 12, is waveform W20. The signal at node 20 is applied to peak detector 14, resulting in a positive peak signal (W22) and a negative peak signal (W24), which are averaged (W26) and used as the detection threshold, which comparator 16 compares with the amplified input signal from node 20. Comparator 16 outputs a logical "1" at node 28 if node 20 is more positive than the sum signal at node 26, otherwise it outputs a logical "0". The output of comparator 16 changes when the signal at node 20 is about halfway between its positive and negative peaks.
Another known detection circuit 30 is shown in FIG. 3, with the waveforms shown in FIG. 4. While detection circuit 30 is more complex than decision circuit 10 shown in FIG. 1, it has additional capabilities. For example, detection circuit 30 has peak detectors 31 which are resetable. Each peak detector 31 acquires an updated peak value after each positive or negative transition of the output signal. Each peak detector 31 has a comparator 34 with a small hysteresis to prevent oscillations near the switching point. As with detection circuit 10, the input signal is amplified by an amplifier 32, and the outputs of peak detectors 31 are averaged at node 56 and used as the threshold voltage for comparator 38. The output of detection circuit 30 is at node 60. That output is also used to reset the switches in peak detectors 31, as the output signal at node 60 is fed to edge detectors 42 (one directly and one after being inverted by an inverter 40) coupled to the switches.
Waveforms at nodes 50, 52, 54, 56, 58, 60, 62 and 64 of detection circuit 30 are shown in FIG. 4 as waveforms W50, W52, W54, W56, W58, W60, W62 and W64, respectively.
Yet another detection circuit 70 is shown in FIG. 5, with associated waveforms shown in FIG. 6. Detection circuit provides an output response with less delay than other detection circuits, and has better transition detection, but requires a noise-free environment. The increased noise sensitivity comes from a peaking circuit 82, which is needed for the improved signal transition detection. Peaking circuit 82 amplifies noise and interference more than the signal. Consequently, at the output of the peaking circuit, the signal-to-noise ratio is much worse than at the input. This makes the circuit unreliable in noisy environments. When the noise is amplified, multiple transitions might be spuriously detected at transition points, such as t1-t5 shown in FIG. 6., where only single transitions should have been detected.
From the above it is seen that an improved detection circuit is needed.